Conventional electrolytes for aluminum electrolytic capacitors generally comprise some type of ionizable compound dissolved in a solvent.
The chemical or electric properties of the electrolyte, such as electric conductivity and scintillation voltage become main factors for determining the electric properties of the electrolytic capacitors.
For example, a solution of boric acid or ammonium borate in ethylene glycol, which has low electric conductivity and high scintillation voltage, has been used for aluminum electrolytic capacitors operating of 200 V or higher (U.S. Pat. No. 1,891,207). The maximum operating temperature for such an electrolyte system is less than 100.degree. C. and normally 85.degree. C. because a large amount of water formed during the rapid esterification of glycol and borate species at higher temperature deteriorates the electric properties of the electrolytic capacitors. The minimum operating temperature in such a system is above -20.degree. C. since glycol freezes at -17.degree. C.
On the other hand, a solution of an amine or an quaternary ammonium salt of carboxylic acid in an aprotic solvent such as N,N-dimethylformamide or .gamma.-butyrolactone is used for low voltage electrolytic capacitors, which have a good low temperature characteristics (U.S. Pat. Nos. 3,812,039 and 4,473,864).
However, electrolytes which have higher electric conductivity over the wider temperature range are desirable for imparting lower equivalent series resistance (ESR) and lower impedance at high frequencies to the electrolytic capacitor containing them.